This invention relates to links for grate conveyors chains and a method of manufacturing the links.
The links to which this invention is directed are primarily for use as the links of chains for grate conveyors such as disclosed in U.S. Pat. No. 3,735,858issued May 29, 1973, more particularly for a grate conveyor of the type referred to as a cooling grate conveyor for the cooling section of sintering apparatus in which iron ore, flux and solid fuel substances are continuously delivered upon the conveyor, ignited at one end, and fed forward on the conveyor, air being drawn down through the bed of material on the conveyor (i.e. through the conveyor grates) for some distance in the course of its travel to complete the sintering and partial fusion of the product.
In the prior art relating to grate conveyor chains, the pivotally connected links of the conveyor chains to which the grate members are connected are conventionally made of one-piece metal castings, including a pair of transversely spaced side walls formed at one of their ends to define a yoke portion, the chain bar sides merging at their opposite ends to define a neck or lug portion, the prior art chain link also including an integrally cast thin wall arched or web cover. The lug and the yoke portions contain circular connecting holes located on a transverse axis. The structure just described is one link in the prior art chain, and it will be understood that a plurality of such links are pivotally connected to each other with the neck portion of one chain link being received within the yoke portion of the next succeeding contiguous chain link. The plurality of pivotally connected chain links are known in the art as a "chain strand."
In the one-piece chain link construction of the prior art as just briefly described, a serious problem has arisen because of changes in operating temperatures and fluxing methods. In particular, the leading and trailing faces of the link holes wear from abrasion. In addition, the holes in the links tend to "egg" out in the direction of travel because the metal creeps or stretches at operating temperatures due to mechanical pulling tensions applied to the chain during normal movement of the conveyor and when the conveyor moves around an end sprocket or the like, or during the return run of the conveyor. Also, the links are attacked by the hot gases so that they tend to "scale" and lose thickness. Further, in the final stages of the sinter, temperatures as high as 2200.degree. F. on the top of the product bed are encountered, and the links probably reach temperatures of the order of 1000.degree. F. to perhaps 1500.degree. F. As a result of these various conditions, the connecting portions of the links often fail by rupture through the wall sections of the connecting holes, generally in a transverse plane of the link and passing through the minimum thickness of the wall section of the link surrounding the hole.
Various ways of minimizing the rupture problem have been suggested. For example, some sintering machine manufacturer's have experimented with links made of SFSA-ACI castings of the ferritic straight iron-chromium types, but these have been disappointing due to their very poor hot strength. There have also been tests using the SFSA-ACI type HP, which contains nominally 35% Ni, 26% Cr and the balance mainly iron. This alloy has the highest hot strength of all of the ACI alloys, but is also very expensive due to its high Ni content. The most widely employed link alloy today is ACI type HF, which contains about 20% Cr, 10% Ni and the remainder mostly iron. Nevertheless, while this alloy has moderately good hot strength at operating temperatures it also shows a tendency to wear out in the connecting holes.
Another suggested solution to reduce the wear problem has been to employ hardened cylindric bushings or inserts of various steels in the connecting holes. In that approach the holes and the inserts have been machined to size. Unfortunately, this method is not only costly but also reduces the section thickness of the links around the bushings, so that the links tend to elongate and eventually rupture through the reduced wall sections. Even if rupture does not occur elongation causes misalignment of the links and gaps in the grate.
Attempts have also been made to reduce machining costs by placing rough, unmachined cylindric inserts in the mold prior to casting of the links. It was anticipated that the cast metal would fuse to the bushing during the casting process. The final internal hole could then be finish machined. This approach also failed because the solid insert bushing is heating and expanding while the casting around it attempts to cool and shrink. The walls of the casting tend to crack around the bushing during the cooling process.
Further, even those links that appear to be sound still tend to fail in service by rupturing through the wall sections that have been thinned by the presence of the bushings.